We currently manufacture a cable similar to the one described and claimed in British Patent Application No. 8229562 (C.S. Parfree et al 20) (Serial No. 2128358). The optical fibre package comprises a plurality of secondary coated optical fibres e.g. coated with Nylon 12 (RTM) over a primary Sylguard (RTM) coating which are helically laid up around a kingwire and held in a package by a whipping. This has proved satisfactory, but with the advent of acrylate coated fibres i.e. fibres with one or more layers of acrylic coating over the bare fibre, which are now in widescale production, attempts to substitute these fibres for the Nylon coated fibres failed because the transmission loss was unacceptably high. This was due to the lack of buffering between the acrylate fibres and the rest of the package.
One arrangement for a cable element for use in a submarine cable is proposed in our co-pending patent application (as yet unpublished at the time of making the present application) S.R. Barnes et al 4-3-2 (British Patent Application No. 8516290) in which acrylate coated fibres are drawn through a forming die together with a central kingwire, and a thermoplastic polymer, particularly one marketed under the trade name Hytrel, is extruded around and between the fibres and kingwire in a "one-shot" operation. This encapsulates the fibres and kingwire in a straight lay configuration in one pass without any subsequent extrusion process and the element so formed can be used directly in the longitudinal cavity of a submarine cable such as the one described in Parfree 20, in place of the optical fibre package there described.
This technique is difficult to control in such a way that the original low loss of the fibres is guaranteed in the finished element and stresses are imparted to the fibres as they pass through the extrusion head which are difficult to control or quantify.
Whilst this has proved suitable for relatively short lengths e.g. up to 5 Km, the losses and the yield are still not as good as we have obtained with the previous package referred to above.
An alternative approach is disclosed in our published British Patent 2113903B (L.R. Spicer 26) in which optical fibres and a coated string are pulled through precision tooling having a bore for each fibre, whereby the optical fibres are partially embedded in the outer periphery of low-density-polyethelene-coated string which has been heat softened, and extruding an insulating sheath over the partially embedded fibres, which sheath can for example be high density polyethelene.
None of the proposals discussed above enables long lengths e.g. 50 km of cable to be produced in a continuous process. For example using the above processes we achieved only 5 kms before a fibre broke due to dust build up in the tooling. Also spliced fibres have a bulge in the coating so they would not pass through the precision tooling of e.g. 2113903B.
Another proposal is set forth in published British Patent Specification 2136350A in which a central strength member is heated and a first layer of thermoplastic elastomer is extruded onto the heated central strength member. Optical fibres are laid along a helical path onto the first layer with a planetary motion. A second layer of thermoplastic elastomer is extruded over the fibres and merges with the first layer, and then a protective nylon sheath is extruded around the elastomer. For undersea applications the central strength member may be a central conductor of a coaxial cable, for low frequency signaling of information for surveillance, maintenance and control.
However the equipment is complex requiring as it does a rotating die and rotating bobbins and lay plate equipment (FIGS. 3 and 4) requiring a completely enclosed environment for each fibre during passage from the bobbin to the die. Furthermore each fibre passes through four separate guides during its passage from the supply bobbin to the closing die.
It is an object of the present invention to overcome the above problems in a simple effective manner to provide a cable element of very long length e.g. 50 kms and above, suitable for a submarine cable and using acrylate or equivalent coated fibres.